[itsme]

Hi there,

I think I have finally decided what engine I am going to have a go at building. I was thinking about designing my own from scratch, and even had thoughts about a DKM type engine, but I have come up with a better idea. I am going to basically build the SW92, BUT with a few changes to the design.

The biggest difference will be that my engine will use a proper epitrochoid rather than 2 circles. The porting may have to be adjusted slightly to suit the epitrochoid. I am going to keep an eccentricity of 3.5mm, however I am going to decrease the rotor radius from 28mm to 25.5mm. 28mm seems too big for this engine. I'm not sure if this has something to do with the housing that was used. The rotor width will stay at 18mm. After these changes have been made, the capacity of the 'new' engine is 8.4cc. I will therefore be making an SW84, not an SW92 (I may as well change the name slightly while I'm at it...). The only other change I'm going to make is to the cooling fins. I prefer the look of the OS style fins to the ones on the plans. Oh, I'll also replace the homemade venturi with a commercially available carburettor.

This should save a lot of time, as the basic design is already in place, so I can concentrate on the more important parts.

Mark, maybe you can help me here on some detail. I was looking at the plans for the SW92 and it looks like the internal gear is cut into the rotor. It seems like quite a good idea to do this. Am I correct in thinking this? Finally, does the author give any detail on how he actually cut that internal gear in any of the text? It's at times like this that I wish I could read German...

Thanks
Warren

[RotarySMP]

Hi Warren,

I speak german, but the notes in the text are very limited. I have read these instructions a few times and am of the opinion that the were written for an early model of this engine, or the drawings are missing some detail.. Note that the photo top right also shows an motor with OS style cooling fins. In the assembly instructions it mentions - "The stationary gear is secured with four srews. It has 14 teeth , the holes are not symetrically distributed. The screws must be peaned to sercure them." This is not illustrated in #8 (the stationary gear) or part four, the side housing, each of which should have four M2 holes not evenly distributed. I think you are going to have to look at how the phasing gears mesh on a Mazda and align this the same.


On the rotor it says:

The cuts in the rotor must be evenly distributed, so that the apex seals function precisely. The given angles must be maintained, and the axis of symmetry must cross the center line. Through lapping of the sides of the rotor, using lapping paste on a asurface plate, you are aiming for the following tolerance: The middle of the rotor should be 4 to 5 microns wider that the corners.

The author gives no info on cutting that internal gear, but you will just have to grind up a form tool (I don't know how to ally the figures, but that box under the rotor defines the gear shape. I'll take my plans to work and ask one of the guys to explain it.)

Then you will need to make a shaper attachment for your lathe, and a way to index 21 on your lathe spindle.

[itsme]

Hi,

Getting the lathe spindle to index could be a bit of a challenge. I was thinking about maybe making a cutter (like an involute cutter) with the correctly shaped teeth for an internal gear. I would then use this cutter to make a broach, which would look similar to a spur gear, but have many steps getting larger on each step. Once this is done, I could then use some kind of press to press the broach through the hole and at the end of it, I should have an internal gear.

I don't think that the shape of an internal gear is the same as the 'negative' of a spur gear, otherwise I could just use a normal involute cutter to make the broach. On the other hand, I don't think the shape is that far off. I reckon a fairly accurate gear can be made at home, but I don't think it will ever be perfect.

Although the plans don't give much detail on the fixing of the spur gear, it does show the screw that is used. I think it is part 26. I've been looking at the plans, and still can't quite figure out why those screws aren't symmetrically placed. Is there something that I'm missing. It would just make sense to place them 90 degrees apart. I guess I can always make it up as I go along...as long as it works in the end.

Regards
Warren

[Mat-C]

Have you checked out available planetary gearboxes? Small/cheap cordless screwdrivers often use these because it is easy to stack multiple stages axially.
You probably know about these, but for those that don't: a planetary gearbox has a driven centre gear with 2 or 3 gears revolving around this, meshing with the driven gear and an inverse outer gear, the (slower) drive being taken off the mounting of the revolving cogs.

http://www.du.edu/~jcalvert/tech/planet.htm (diagram is middle-left).

[itsme]

Hi Mat-C,

I had never thought of that. I have a cheap cordless drill that I might have a look in today.

There are, however, a few problems that come to mind with this. One problem is the size of the gear. You either need to get lucky and find one that is just right for the engine design, or design the whole engine around the gear (!!!). Another is the material. I'm not sure what they are made of in the drills, but if it's plastic or something like that, it won't do.

You probably aren't familiar with the engine design that we are talking about, but on this design, the gear is actually cut into the rotor (a permanent fixture). So, for this particular engine, unless we can find a drill that has a gear that's got a rotor on the back of it, it could be a problem.

I'll have a look anyway though, because it would still be useful to find a cheap source for these gears.

Thanks
Warren

[RotarySMP]

You need a 2:3 gear ratio. The SW92 uses a 21 and a 14 tooth, kind of unlikely that a cheap cordless drill has such a low reduction ratio. Also, the SW92 put out about a hp, and the cheap stamped gears from a drill are unlikely to be able to transmit that.

A way of making an oil cooled rotor I have been thinking of, would be to make the rotor in two parts, with the phasing gear cut out of flat plate, and the other part being the other side with the bearing and having the internal oil passages machined in, then furnace brazing the two parts together before machining the outer triangular shape and the seal slots.

Making the broach would work also, but would be a lot of work. This would be the way to go if you are intending to make many, but for just one, shaping would be a lot easier. Also, the pressures required to shape out each tooth are not that high, and can be done on a little lathe. To index 21, you would just need to make a plate on your CNC mill with 21 holes, which can be attached onto the lathe spindle, and then make a little widget to put a pin through those holes.

If you have a mill, drill with a quill, you can also use that to shape the teeth, and a rotary table to index it.

You should be able to draw up the right shape of the tools in CAD. You must be able to CNC grind this tool to shape using the small grinding attachment in the mill.

The four M2x5 screws (part #26) are set into a non symetrical hole pattern to ensure the stationary gear can only be fitted one way, as that is how the rotor is "timed". Unfortunatly, the designer what this alignment is to be.

As the rotor turns at 1/3 E shaft speed, and the number of teeth on the rotor is divisible by three, I would just put a phaseing gear tooth in line with the Apex seal, and on the stationary gear, put a tooth gap aligned with the the major axis of the housing.

Does this sound right to you?

[itsme]

Hi Mark,

I've been thinking about making a broach and I think that your method is much better. It is an excellent idea to grind the tool profile on the mill - I hadn't thought of that. You can buy small, cheap arbor presses on ebay as well (you'll get one for about £30). I thought that instead of using the lathe or quill on the mill, a simple little press like this could be used. I'm sure a rotary table could be mounted to its base and then the rest should be easy.

One problem that I am having, is finding decent information on the shape of internal gear teeth. I'm not entirely sure how the profile is formed. In order to grind a form tool, I'd obviously need to get this profile from somewhere. One method would be to download a 3D model of an internal gear from one of the online distributor-type places, but it would still be useful to know where that shape has come from.

I'm pretty sure that aligning the gears won't be a huge issue. Like you said, you can make the gears with the teeth in a certain position and then just assemble it properly. I still think that you could just use 4 symmetricaly placed holes for the retaining screws. If I'm the only one who will be assembling/disassembling the engine, I'm sure I could figure it out (at least I hope so...).

I also had a look inside that drill this morning. The internal gear was surprisingly well made, but too big for a 'small' wankel. It had a pitch diameter of somewhere around 33mm, but it did have 45 teeth, so a 2:3 ratio would be possible. I think I'll just stick to my own gears for now though...

With a bit of luck, the stuff for my grinding attachment should arrive on Monday, so I can then get on with that. I'm also trying to scrape up some money to buy ballscrews for my mill, because I've just got this feeling that the standard screws aren't going to keep the tight tolerances forever, particularly if I'm machining lots of engine parts (which I will be).

Regards
Warren

[RotarySMP]

Hi Warren.

I took an hour and translated the instructions for the SW92 for you, and my bloody computor crashed just when I was finished.

I'll try and redo it this week.

[itsme]

Hi Mark,

That would be very kind of you to translate the instructions - I'm sure it would be a great help. Don't you just love it when your computer doesn't do what it's meant to? Hehehe.

I managed to download a CAD model of a 60 tooth, module 1 internal gear. I'm not sure if the tooth shape would be very different to a 21 tooth gear, but I have at least managed to extract the important shapes from it to design a form tool. I've also been thinking about how I can lock the spindle on my mill so that I can shape the teeth using the quill. I think I have a plan to do that using an attachment that locks the spindle from the top via the drawbar. This machine is going to look nothing like a Sieg X1 by the time I've added all these extra attachments to it...

Regards
Warren

[itsme]

Hello,

This morning I went out and bought what will hopefully be a running wankel engine in a few months. What you can see is some cast iron, aluminium and steel. I can't say that I'm looking forward to turning that big lump of iron down and I'm also having second thoughts about the OS-style cooling fins. Maybe the fins on the plans will do for a first engine. Machining should start soon...

Regards
Warren

[RotarySMP]

Hi Warren, Where do you buy your C.I? I was meaing to get some when I was at Cranfield last month, but bought pickled onions, Cheddar (2.5KG) and Cadburies instead.

Iron is nice to turn, as long as you are using CNC and don't have the black crap working into your pores

[itsme]

Hi Mark,

I buy most of my metal from a place called Metal Supermarkets.

http://www.metalsupermarkets.com/

It's a big chain that has branches all over the place. If you have a look at their site in the 'store finder', you'll see that they also have branches in Austria. I have no idea if those are close to you or not. The one near me is very helpful and friendly and they will sell you metal in any quantity, which is very useful when you don't need 3m of cast iron.

Sadly, I'm going to have to turn that iron down with a non-CNCed Emco Compact 5. It's a great little machine, but when you start machining 105mm diameter cast iron, it's pushing it a bit. We do have an Emco Compact 8 sitting around (as you do...), but it has no stand, so I can't really use it - damn!!!

It's funny you should mention that black crap. I decided to cut through a piece of that cast iron this afternoon using a hacksaw. It probably wasn't such a good idea. It took me over an hour of continuous cutting to get through it (105mm diameter) and I looked like a coal miner by the end of it. I'm not sure how my face ended up getting covered in that stuff, but it was. It's not something I'm going to do again in a great hurry...

I still haven't got my rotary tool yet - hopefully tomorrow, then the building can begin.

Regards
Warren

[RotarySMP]

Hi Warren,

Yes we have the metal super market, but our one doesn't stock cast iron. Do you also get the feeling from yours that the the price is determined by what ever they think they can screw you for? I find going there and buying, without first calling an bargaining a price is a recipe for going to the cleaners.

Those Emco's are nice machines. Much nicer than the 7x12 SIEG I use at present.

[itsme]

Hi Mark,

The Metal Supermarket near me is very friendly and I've also got to know the guys there, which always helps. I have been to a different one though and besides the guy almost being too lazy to even cut the metal, I think he also rounded all the prices up to the nearest 10. I have also experienced that 'anti-hobbiest attitude' with other suppliers though. Another place that I came across on the net is http://www.mallardmetals.co.uk/ . I haven't used them, but their prices seem reasonable and they are listed in their catalogue, so they can't argue.

The Emcos are excellent machines - the only problem is the price. My Dad managed to get both of these machines almost for free through industrial-sized dealers who really had no idea what the true value of such a 'small' machine is.

I have also set the Compact 8 up on the workbench temporarily, because the the 5 really can't cope with that iron. Machining should start tomorrow with basic facing and getting the metal to size. Still waiting for the grinding parts...

Regards
Warren

[itsme]

Hi there,

Quite a lot happened today. I now have the capability to grind an epitrochoid on my mill. Hooray!!! The 'conversion' to high speed spindle took roughly 5 hours to do. It can all be taken off the machine and put back on in less than 5 minutes.

I built a flexible extension from a rotary tool into an MT2 arbor. It uses 2 ball bearings to support the 'new' spindle and everything is contained within the X1's own spindle. I made a custom drawbar that is hollow so that the shaft can run through it. The rotary tool mount is temporary (just in case you were wondering...). I haven't tested it out on any metal yet - that should happen in the next day or two.

I have also included some photos, the last one is a general shot of the workshop.

Regards
Warren

[RotarySMP]

Hi Warren,

You have made a nice job of that grinding attachment. Don't forget to dress the wheel before you use it, as I have found the dremel wheels to be pretty out of round from the manufacturer. Until dressed, the vibration caused a horrible finish.

You have a great set up there. That milling attachment on the C5 will come in handle for cross drilling and milling the key slot in the E-shaft.

[RotarySMP]

Here is the tanslation of the instructions I promised:

The first bit is just Blah Blah. There is a reference to FMT magazine edition 97, which likely has some Wankel information.

The SW92 was designed in 1960 – 1962 by Dipl Ing Julian Faleki of Poland, and has the following specs:

Swept volume……………………………......9.2 CC
Compression ……………………………….......7.4:1
Max Power output……………..………......1.5 HP
Power at 12000rpm…………….………....……1HP
Fuel…………………………….........Meth ylalchohol
Lubricant……………………....10-20% Caster oil
Glow Plugs…………………………......…Start 1.5V
Weight………………………………......� �.900-930g
Eccentricity…………………………......� �….3.5mm
Major Radius…………………………......…….2 8mm
Rotor thickness………………………....…….18m m
Apex seal replacement interval……….….30 H

Building Instructions.

[ 1 ] Rotor Housing.
Study the plans carefully for the location of the housing screw holes. The sharp edges of the rotor housing are manufactured by first machining the housing 1-2mm wider than plan then, after chroming, they are ground to dimension.
The inlet and exhaust are radiused to exactly 0.5mm. It is important that the M4 holes are carefully drilled with equal spacing, especially in the area of cut B-B. The countersink diameter should not exceed 5mm. Accuracy is extremely important, otherwise sealing and compression will suffer.

[ 2 ] Rotor
The seal slots must be symmetrical to ensure good sealing. The given angles must be maintained, and the axis of symmetry must cross the centre line. Through lapping of the sides of the rotor, using lapping paste on a surface plate, you are aiming for the following tolerance: The middle of the rotor should be 4 to 5 microns wider that the corners.

[ 3 /4 ] Front and Rear Housing

Here it is important to follow the correct order of set ups. First you turn a ring of OD ~110mm ID 90 (–0.02mm) about 30mm wide. This ring is the assembly jig. Press the phasing gear [8] in the rear housing, and then insert the bearing. Install the needles [16], the rotor [2] with tensioners [11], springs [10] and apex seals [10] ontothe E-shaft [5]. The rotating assy is inserted into the rotor housing, and the end housings fitted. Slip the assembly jig ring over the housings, then rotate the rear housing until the E shaft turns freely in both directions without any binding. Now the 4mm holes are bored in the end housings with the jig ring in place. After disassembly, the holes are enlarged to 4.2mm and the R45 finished.



Some Assembly Hints.
Before assembly, the entire rotating assembly should be statically balanced. This is performed on pair of knife edges.
The rotor and needles are installed on the E-shaft with plenty of grease. The forward counterweight [6] with it’s key [18] and rear counterweight [7] with it’s pins [20]. The Phasing gear [8] is secured with 4 screws. It has teeth. The mounting holes are not symmetrical. These bolts should be safetied by peening . The bearing [35] is fitted to the rear housing, with sealing rings [15] on each side, and secured with the nut [13]. The front main bearing [36] with it’s seals [19] is fitted to the forward housing, secured by a nut [12], which must be safetied with a split pin.

Both bearings should be lubricated with caster oil. The exhaust [33] and intake [27] are secured with set screws [34]. The spray bar needle [30] is soldered into the knurled thumbscrew [29], such that the 0.9mm hole in the spray bar can be completely closed.

The main housing screws [24] and mount screw [25] are torqued evenly in a cross pattern. If your work is of sufficient quality, no gaskets or sealant will be necessary.

Starting.
First check that the engine turns over without binding. The compression is checked once the glow plug is installed. The resistance to rotation should be less than equivalent piston engine. For the first runs, choose a prop of around 250m diameter. The fuel tank must be above the level of the spray bar. During running in, a fuel of three parts methylalchohol to one part caster oil by volume is appropriate. You will need a conventional starter giving 3000 – 4000 rpm, although a bend grinder can also be modified for this purpose.

Troubleshooting.
If the motor requires greater starter speed to start than mentioned above, then the rotor to side housing clearance is too loose. On the other hand, if the rotor to side housing clearance is too tight, it can miss, or nip up in service. These symptoms can also occur due to excessively lean mixture, or a propeller of insufficient mass. With insufficient cooling, the motor will tend to miss. If the rotor to side housing clearance is correct, then the motor will run in relatively quickly. It is considered run in when it runs without hesitation to 12000 rpm. The fuel-oil mixture can then be altered to 10-15% caster oil, whereby the higher the percentage of oil, the easier the engine is to start, at lower starter RPM.

[itsme]

Hi Mark,

Thanks for the translation - it should be a great help.

I tried the grinder out yesterday on a piece of cast iron. I dressed the wheel as you suggested and it definitely needed it - it was like an eccentric shaft! At first I was getting a fairly poor finish. It was all 'stripey' and if you ran your finger nail across it, it was quite rough. I then moved onto a bigger diameter wheel that was finer and it sorted the problems. I think the finish is suitable for a running engine. Life is easy with CNC...

Thanks again
Warren

[RotarySMP]

I'm glad you could get a set up with your grinder which gives good results.

Considering the plans call for the medium carbon steel apex seals to run on chromed steel surface, it will be interesting to see your results with a Cast-iron surface.

Once you have the rotor finished and need to grind the sides smooth and provide the 4-5micron clearance at the tips you might want to consider making an arbor to hold it in the chuck and press it onto a peice of very fine sand paper, sitting on balsa wood. I have used this method to polish rivet snaps which have a slight crown. The internal roller bearing surface is to be case hardened to HRC 58 for a depth of around 1mm. The sides of the rotor are chromed.

note from the RH side of the plans that the E shaft bearing surface is to be case hardened to HRC 58 to a depth of about 1mm.

The Apex seal is nominally 1mm thick, and there should be a 0.01 - 0.03 clearance between seal and slot. The length os the apex seal should be 0.03 - 0.05 shorter than the rotor housing width.

[itsme]

Hi there,

This weekend I've been working on the housing for my engine and it is just about finished. With the CNC running, it was quicker than I thought it would be.

I started with with a lump of 105mm diameter cast iron and turned it down on the lathe to 95mm. I then cut the fins 8mm deep using a parting-off tool. Unlike the drawings, I decided to keep the fins and the 'inner part' of the housing concentric (it is much easier this way). Once this was done, I moved onto the mill, where I cut out the epitrochoid using a 6mm endmill. Whilst the housing was still setup on the mill, I also ground it and drilled all the holes so that they are all in the correct position relative to the centre of the epitrochoid. I then set the housing up vertically to machine the ports and the glow plug hole.

There are still a few things that I need to do to finish the housing off. The 0.5mm radius hasn't been added to the ports yet (where they enter the housing), the holes for the intake and exhaust tube retaining screws/pins haven't been drilled and the sides need to be lapped. I think I'm going to find a piece of fairly thick glass to finish off the side surfaces (using abrasives).

I will probably start on the rotor next. I'm going to have a look to see if I can find a standard needle roller bearing to fit it, otherwise I'll just make my own. If I can find one to fit, it'll save me trying to harden the rotor. I'm also trying to decide what material I should make the apex seals out of. I would like to use something that is fairly soft. Any suggestions?

Regards
Warren